This invention relates generally to the rendering of latent electrostatic images visible using multiple colors of dry toner or developer and, more particularly, to creating highlight color and/or custom color images on a single image receiver using relatively high development fields.
The invention can be utilized in such imaging technologies as xerography and ionography. In the practice of conventional xerography, it is the general procedure to form electrostatic latent images on a xerographic surface by first uniformly charging a photoconductive insulating surface or photoreceptor. The charge is selectively dissipated in accordance with a pattern of activating radiation corresponding to original images. The selective dissipation of the charge leaves a latent charge pattern on the imaging surface corresponding to the areas not struck by radiation.
This charge pattern is made visible by developing it with toner. The toner is generally a electrically charged, colored powder which adheres to the charge pattern by electrostatic attraction. The developed image is then fixed to the imaging surface or is transferred to a receiving substrate such as plain paper to which it is fixed by suitable fusing techniques.
Recent developments in the art of xerography have been directed to highlight color imaging wherein at least two colored images are produced in a single pass. Several concepts for xerographic single pass highlight color (SPHLC) imaging systems are known. One of the more elegant and practical of these is tri-level imaging. In general in tri-level imaging, two different latent images are formed in one imaging step, with a white or background level at an intermediate voltage. With the development bias near the white level in either case, one image is charged-area developed while the other is discharged-area developed. This is accomplished by using positive toner for one color and negative toner for the other, in separate housings. Typically one toner is black and the other is a preferred color for highlighting.
The concept of tri-level xerography is described in U.S. Pat. No. 4,078,929 issued in the name of Gundlach. The patent to Gundlach teaches the use of tri-level xerography as a means to achieve single-pass highlight color imaging. As disclosed therein, the charge pattern is developed with toner particles of first and second colors. The toner particles of one of the colors are positively charged and the toner particles of the other color are negatively charged. In one embodiment, the toner particles are supplied by a developer which comprises a mixture of triboelectrically relatively positive and relatively negative carrier beads. The carrier beads support, respectively, the relatively negative and relatively positive toner particles. Such a developer is generally supplied to the charge pattern by cascading it across the imaging surface supporting the charge pattern. In another embodiment, the toner particles are presented to the charge pattern by a pair of magnetic brushes. Each brush supplies a toner of one color and one charge. In yet another embodiment, the development system is biased to about the background voltage. Such biasing results in a developed image of improved color sharpness.
In tri-level xerography, the xerographic contrast on the charge retentive surface or photoreceptor is divided three, rather than two, ways as is the case in conventional xerography. The photoreceptor is charged, typically to 900 volts. It is exposed imagewise, such that one image corresponding to charged image areas (which are subsequently developed by charged area development, i.e. CAD) stays at the full photoreceptor potential (V.sub.ddp or V.sub.cad, see FIGS. 1a and 1b). The other image is exposed to discharge the photoreceptor to its residual potential, i.e. V.sub.c or V.sub.dad (typically 100 volts) which corresponds to discharged area images that are subsequently developed by discharged-area development (DAD). The background areas are formed by exposing areas of the photoreceptor at V.sub.ddp to reduce the photoreceptor potential to halfway between the V.sub.cad and V.sub.dad potentials, (typically 500 volts) and is referred to as V.sub.w or V.sub.white. The CAD developer is typically biased about 100 volts closer to V.sub.cad than V.sub.white (about 600 volts), and the DAD developer system is biased about 100 volts closer to V.sub.dad than V.sub.white (about 400 volts).
U.S. Pat. No. 4,913,348 granted to Dan A. Hays on Apr. 3, 1990 discloses an imaging apparatus wherein an electrostatic charge pattern is formed on a charge retentive surface. The charge pattern comprises charged image areas and discharged background areas. The fully charged image areas are at a voltage level of approximately -500 volts and the background is at a voltage level of approximately -100 volts. A spatial portion of the image area is used to form a first image with a narrow development zone while other spatial portions are used to form other images which are distinct from the first image in some physical property such as color or magnetic state. The development is rapidly turned on and off by a combination of AC and DC electrical switching. Thus, high spatial resolution multi-color development in the process direction can be obtained in a single pass of the charge retentive surface through the processing stations of a copying or printing apparatus. Also, since the voltages representing all images are at the same voltage polarity unipolar toner can be employed. In order to effect development of all images with a unipolar toner, each of the development system structures is capable of selective actuation without physical movement.
There is an increasing interest in single pass custom color (SPCC). Custom color differs from highlight color in two ways. First, it generally refers to a very specific color, "customized" for a given customer or user. The customer typically will be very concerned that the hue meets his specifications. Thus, the specific color toner should be formulated in the factory rather than created by the process, as it is in process color systems, unless there is extremely good process control. Secondly, it is typically used to provide an instant identification of the document with the customer and with the customer's advertising. It would not be the color desired for normal highlighting. Ideally, it is desirable to provide SPHLC and SPCC on the same document, that is, to enable documents to be printed with both a custom color and a highlight color, along with black, in only one pass through the system. Unfortunately, tri-level is available only for two colors corresponding to the two polarities of electrical charge. Other known concepts are less elegant and introduce other problems.
U.S. Pat. No. 4,731,634 granted to Howard M. Stark on Mar. 15, 1988 discloses the method and apparatus for rendering latent electrostatic images visible using multiple colors of dry toner or developer and more particularly to printing toner images in black and at least two highlighting colors in a single pass of the imaging surface through the processing areas of the printing apparatus. Two of the toners are attracted to only one charge level on the charge retentive surface providing black and one highlight color image while two toners are attracted to another charge level to form the second highlight color image.
In order to provide two highlight color images, the device disclosed in the '634 patent must rely on very low development fields which are difficult to control. This is because the contrast voltage normally available in standard xerography is divided four rather than two ways as in conventional xerography and three ways as in tri-level, single pass, highlight color xerography.